Generally, the cell, such as an AAL2 or AAL5, of the ATM protocol used in a CDMA system consists of a 5-byte header field and a 48-byte data field (as shown in FIG. 1). It is used to communicate multimedia traffic, including voice and data traffic, through a backbone network.
An AAL5 cell of the ATM protocol contains an 8-byte trailer appended to the traffic data when converting voice traffic or data traffic signal into the cell. In a prior CDMA system, the 8-byte trailer has been widely used to communicate the traffic data because it provides an efficient process for converting the data traffic signal. However, appending the 8-byte trailer may become burdensome for the AAL5 cell when a voice signal is processed with the AAL5 cell. While each frame of half, quarter and one-eighth rate voice traffic data may be processed with one AAL5 cell, two AAL5 cells are needed to communicate the full-rate voice traffic data (i.e. a 24-byte data frame). This is because 56 bytes are required for the full-rate data (i.e., a sum of 5 (header), 24 (voice data), 19 (control data), and 8 (trailer) bytes). This may result in traffic overload. Because a base station transmission system (BTS) and a base station controller (BSC) are synchronized by means of 20 ms sync signal, the second cell of the two AAL5 cells having the fall-rate voice data may be lost (i.e., the cell may be discarded) during a communication between the BTS and BCS if the first cell comes in and then the second cell comes in after the sync signal for the first cell arrives.
Also, when a transformation between an AA5 cell and an AAL2 cell is to be made in order to communicate between the BTS and the BSC using the ATM cell, an AAL5 cell consists of 5 bytes of a header field, 24 bytes of a voice or data traffic field, a Control-Data field, 1 byte of a HEC field for error detection, and a PAD field used to pad meaningless data (as shown in FIG. 2). An AAL2 cell consists of 5 bytes of a header field, 1 byte of a START field (composed of a 6-bit Offset field, a 1-bit Sequence-Number field and 1-bit parity bit field), 48 bytes of a voice or data traffic field, and a PAD field (as shown in FIG. 3).
Therefore, when communication is performed from the BTS to the BSC through the AAL5/AAL2 cells, converting a full-rate voice traffic data into an AAL5 cell at the BSC may cause not only traffic overload, but also cell loss due to the discard by the sync signal at the BSC that attempts to decode the cell data. In addition, communication between BTS and BSC through AAL5/AAL2 produces another problem, that is, a separator is necessary to separate each voice traffic data in the process of decoding AAL2 cell data converted from many of AAL5 cells.